The present invention relates to a metering system and, more particularly, to a modular meter for measuring electricity consumption by a plurality of loads.
The total power consumption of a building or other facility is monitored by the electric utility with a power meter located between the utility's distribution transformer and the facility's power distribution panel. However, in many instances it is desirable to sub-meter or attribute the facility's power usage and cost to different occupancies, buildings, departments, or cost centers within the facility or to monitor the power consumption of individual loads or groups of loads, such as motors, lighting, heating units, cooling units, machinery, etc. These single phase or multi-phase electrical loads are typically connected to one or more of the branch circuits that extend from the facility's power distribution panel. While a power meter may be installed at any location between a load and the distribution panel, typically a power meter capable of monitoring a plurality of circuits is installed proximate the power distribution panel to provide centralized monitoring of the various loads powered from the panel.
Flexibility has favored adoption of digital power meters incorporating data processing systems that can monitor a plurality of circuits and determine a number of parameters related to electricity consumption. A digital power meter for measuring electricity consumption by respective branch circuits comprises a plurality of voltage and current transducers that are periodically read by a data processing unit which, in a typical digital power meter, comprises one or more microprocessors or digital signal processors (DSP). The data processing unit periodically reads and stores the outputs of the transducers quantifying the magnitudes of current and voltage samples and, using that data, calculates the current, voltage, power, and other electrical parameters, such as active power, apparent power and reactive power, that quantify electricity distribution and consumption. The calculated parameters are typically output to a display for immediate viewing or transmitted from the meter's communications interface to another data processing system, such as a building management computer for remote display or further processing, for example formulating instructions to automated building equipment.
The voltage transducers of digital power meters commonly comprise a voltage divider network that is connected to a conductor in which the voltage will be measured. The power distribution panel provides a convenient location for connecting the voltage transducers because typically each phase of the power is delivered to the power distribution panel on a separate bus bar and the voltage and phase is the same for all loads attached to the respective bus bar. Interconnection of a voltage transducer and the facility's wiring is facilitated by wiring connections in the power distribution panel, however, the voltage transducer(s) can be interconnected anywhere in the wiring that connects the supply and a load, including at the load's terminals.
The current transducers of digital power meters typically comprise current transformers that encircle the respective power cables that connect each branch circuit to the bus bar(s) of the distribution panel. A current transformer typically comprises multiple turns of wire wrapped around the cross-section of a toroidal core. The power cable conducting the load current is passed through the aperture in the center of the toroidal core and constitutes the primary winding of the transformer and the wire wrapped around the cross-section of the core comprises the secondary winding of the transformer. Current flowing in the primary winding (primary current) induces a secondary voltage and current in the secondary winding which is quantitatively related to the current in the primary winding. The secondary winding is typically connected to a resistor network and the magnitude of the primary current can be determined from the amplitude of the voltage at the output of the resistor network. To measure the power consumed by a plurality of loads making up a facility, a current transformer must be installed encircling each conductor in which the current will be measured. Bowman et al., U.S. Pat. No. 6,937,003 B2, discloses a power monitoring system that includes a plurality of current transformers mounted on a common support facilitating installation of a power meter in an electrical distribution panel.
Accurate measurement of electric power also requires compensation for error introduced by the transducers comprising the power meter. For example, the secondary current of a current transformer is ideally equal to the load current in the power cable (the primary winding) divided by the number of turns in the secondary winding. However, magnetization of the core of the transformer produces ratio and phase errors which may vary with the magnitude of the current being measured and the configuration of the particular transformer, including factors such as core material and turns ratio. Typically, error compensation factors are ascertained by experimentation with sample transformers of each production batch and the compensation factors for correcting the calculated output of the meter are stored in a memory in the power meter for use by the data processing unit during calculation of the meter's output.
While initial installation of a power meter at the distribution panel is simplified by integrating a plurality of current transformers into a single assembly, field repairs, modifications and updating of the power meter or the facility's circuitry can be problematic. A power meter is calibrated with a specific set of current and voltage transducers and modification of a meter or replacement of a failed transducer requires recalibration of the meter. A field repairperson typically does not have the equipment necessary to recalibrate the power meter and store new error correction data or a revised transducer configuration in the power meter's memory. As a result, it may be necessary to install a new, calibrated meter or accept inaccurate readings from a meter that has been altered by repair.
What is desired, therefore, is a electricity meter providing flexible construction, simplified installation and improved serviceability.